题目内容
The largest shark known to us, Megalodon, is extinct. Or is it？ Carcharodon Megalodon, commonly known as Megalodon, is believed to have lived between I million and 5 million years ago and thought to have been 52 feet long. It is (or was) a shark that had a jaw 7 or more feet wide. Fairly recently, there has been some speculation about whether it is extinct or just out of reach. But few people believe that Megalodon has found a home deep in the ocean.
There are many known "Living Fossils": Coelacanth, Sea Cucumbers, Sea Urchins, Lobsters, Sea Stars. The common ones like lobsters and sea urchins are not really looked on as anything amazing. They've been around for thousands of years or more, and axe easily accessible to us. What if they weren't accessible and yet still existed？ We would label them extinct. The discovery of a live Coelacanth, a fish long believed extinct, challenged some scientists' long-held beliefs on extinction. There have been recent discoveries Of incredibly large squid, and deep-sea fish never before seen by scientists.
In the 1960s the U.S. Navy set up underwater microphones around the world to track Soviet submarines. The network, known as the Sound Surveillance System, still lies deep below the ocean's surface in a layer of water known as the "deep sound channel'. The temperature and pressure of the channel allow sound waves to travel undisturbed. NOAA's Acoustic Monitoring Project has been using the Sound Surveillance System to listen for changes in ocean structure like ocean currents or volcanic activity. Most of the sounds recorded are common and of no concern. One sound, identified in 1977 by U.S. Navy "spy" sensors, was odd. It was obviously a marine animal but the call was more powerful than any of the calls made by any other reported sea creature. It was too big for a whale. Could it be a deep-sea monster？ One possibility was a giant squid, but no one is sure. It was named "Bloop". Could it be Megalodon？ If Megalodon is still alive down in the bottom of the ocean, we may some day soon discover it. Then what？ Deep sea diving will never be the same, that's for sure!
The following is commonly known EXCEPT ________.
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Why did the U.S. Navy set up underwater microphones around the world in the 160s？
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